Sound recording system and track



Oct 0- c. H. CARTWRIGHT 2,217,154

SOUND RECORDING. SYSTEM AND TRACK Filed Oct; 12,1957 2 Sheets-Sheet; 1

j 3. Y I I C-fffwzfl CAEfWE/GHT A! Zorne y Patented Oct. 8, 1940 UNITED STATES PATENT OFFICE Charles Hawley Cartwright, Boston, Mass., assignor to Radio Corporation of America, a corporation of Delaware Application October 12, 1937, Serial No. 168,565

1'! Claims.

This invention relates to electro-optical sound recording and particularly to the recording of a plurality of mutually relates sound tracks on film strips. Many types of sound recording on motion picture film are known in the art, such as variable density, variable area, doubleand single-hum, standard class A, push-pull class A and push-pull class B, etc. The present invention is directed to combinations of certain of such types; namely, the combination of standard class A with class B and the combination of push-pull class A with class B, wherein the essential advantages of each type are obtained and the disadvantages thereof are substantially eliminated.

To better describe the invention, it is desirable to define these pertinent recording systems as follows: A push-pull recording system is one producing two mutually related sound tracks which will produce the original sound when re- 20 produced with a push-pull reproducing system.

Such a reproducing system may essentially include two photocells, one for each component part of the track, the two photocells generating voltages in a push-pull electrical circuit 180 25 out of phase. A push-pull class A sound recording system is one which produces a sound track having two components, each component being a complete record in and of itself but positioned 180 out of phase with respect to the other track transversely of the film, such a track being reproducible in the above-defined reproducing system. The standard class A system is well known as it is essentially one-half of the push-pull class A system, the single sound track being reproducible with a single photocell. A

push-pull class B sound recording system produces a track having two components, one component representing only the positive cycles of the sound waves and the other component, only the negative cycles of the sound waves; or in other words, when produced with a galvanometer modulator, one component represents the galvanometer deflections on one side of its nul or zero position and the other component represents the deflections of the galvanometer on the opposite side of its nul or zero position. There is no standard class B system, since each track is not a complete record. The present invention, therefore, is directed to a recording system or systems combining the standard class A and class B types and the push-pull class A and class B types which produce combination class A and class B sound tracks and combination push-pull class A and class B sound tracks.

Before describing the combination system, a

review of the relative advantages and disadvantages of each type of system and sound track making up the combinations may also be of value. For instance, the push-pull class A type has the advantage of not being susceptible to unbalance conditions in the push-pull reproducing circuit because each component track is in itself a complete record so that any unbalance only affects the amplitude of the reproduced signals and not their wave form. Another advantage of either class A system is in respect to critical azimuth adjustment of the recording aperture with respect to the light-defining slit. For instance, in the well known type of push-pull recording, two light images, reflected from the galvanometer mirror, are projected on a narrow slit, the slit dividing the images near the center of each. It is thus obvious that the slit may vary in angular position or be rotated with respect to the light images without altering the ideal class A relationship of the beams passing through the slit. Another advantage of the class A systems and tracks is that they are not subject to process distortion to the same extent as other types of track, for instance, class B. It is well known that in processing film the high frequencies of low amplitudes are frequently obliterated by failure of the processing to produce a sharp line between the exposed and unexposed portions of the film. However, where a light beam of appreciable proportions exists at the slit, this distortion is reduced.

A disadvantage of the class A systems and tracks is in the fact that, since a large proportion of the beam or beams is projected to the film during times of no signal or at times when the signal has a low amplitude, there is produced a relatively large amount of clear or transparent portion on the film print, thus making the signal-to-noise ratio low and producing a noisy record. To reduce this noise, mechanism is employed such as ,shutters which shutter off the light except for a small beam, these shutters providing a length of beam to accommodate the amplitude of the modulations. Ideally any modulation produced by movement of the noise reduction shutter vanes should produce a modulation which is automatically eliminated in the push-pull reproducer. Actually, it is only possible to increase the shutter speed slightly over that of single-track recording before noticeable distortion occurs. Due to this gain in shutter speed, the clear portion of the track for no signay may be somewhat less than that used for standard or single-track recording and, therefore, the gain in noise reduction is proportionally increased.

We can now refer to the advantages and disadvantages of the push-pull class B recording system, this type of system employing two light beams projected on a longitudinal slit, the modulating edges of the light beams just touching the opposite sides of the slit at times of no signal. This type of track is inherently noiseless, since at times of no signal no light reaches the film and thus there is no clear portion on the final print. This is an important advantage of class B recording, as it eliminates noise reduction shutters in their entirety. However, this type of track is subject to the disadvantage of being critical to azimuth adjustment between light images and slit, processing distortion, and unbalance distortion. That is, if the slit is not accurately positioned or is slightly rotated with respect .to the modulating edges of the light beams, the two sound components cannot be properly reproduced since each component is not accurately related to the other and low amplitudes may even be lost. This adjustment is so critical that several arrangements have been devised for accomplishing the adjustment, two of which are shown in U. 8. Patent No. 2,096,576. Also, for high frequencies of low amplitudes the resolution of the processing of the film may blur these high frequencies and even eliminate many of them. Furthermore, since only the two components together form one complete record, they must be combined perfectly to reproduce the original sound. Thus, if any unbalance in the reproducing system occurs, such as mismatched photocells or amplifying tubes, distortion is obtained.

The present invention is, therefore, directed to a system which has the essential advantages of the above systems and substantially eliminates the disadvantages thereof. In brief, this recording system is a pure class A system and produces a pure standard class A or push-pull class A record for low modulations up to a predetermined level and is then a combination of either standard or push-pull class A and class B for the higher modulations producing respective combinations of class A and class B records. The overlapping of class B apertures will not, of course, result in the proper combination for the various signal amplitudes. The features thus obtained for the combinations may be enumerated as follows:

1. Ground noise reduction is inherent in the system, thus eliminating noise reduction mechanisms.

2. Azimuth adjustments are not critical since there is no distortion thereof produced for the class A portion and the distortion occurring in the combination portion is at amplitudes where it is not deleterious.

3. Film processing distortion is reduced since it is as low in the class A portion as obtainable iii any type of recording and occurs in the class A-B portion to a lesser degree.

4. It reduces unbalance distortion since there is none in the class A portion and it is considerably reduced in the class A-B portion.

An object of the invention, therefore, is to provide improved push-pull recording systems.

Another object of the invention is to combine the essential advantages of class A and class B recording and either eliminate or reduce to a minimum the disadvantages thereof.

A further object of the invention is to combine push-pull class A and push-pull class B sound recording systems and to produce a combination sound track of this type having the advantages of both types.

A further object of the invention is to produce a sound track, the low levels of which are recorded as pure class A, and the upper amplitude levels I of which are recorded as a combination of class A and class B.

Although the novel features which are believed to be characteristic of this invention are pointed out with particularity inthe claims appended l herewith, the manner of its organization and the mode of its operation will be better understood by referring to the following description read in conjunction with the accompanying drawings forming a part thereof, in which Figure l is a diagrammatic view of a sound recording system involving the invention,

Figure 2 is an enlarged view showing dimensions of the light apertures of Fig. 1,

Figure 3 shows a sound track negative produced with the system of Fig. 1,

Figure 4 shows a mask for recording a combination standard class A and class B sound track,

Figures 5, 6 and 7 show other types of pushpull mask aperture configurations embodying the principles of the invention,

Figure 8 is a graph showing the relationship between the deflections of the light beams and the configurations of the apertures of Fig. 2, and

Figures 9 and 10 are graphs showing the relative distortions between class B and class A-B recording system and tracks produced thereby.

Referring now to Fla. 1, a light source I projects a beam of light by means of a lens 8 onto a mask I having apertures 8 and I therein. The light passing through the apertures l and I is projected by a lens III upon the mirror of a galvanometer ll adapted to oscillate the light beams as shown by the arrow, which is longitudinally of the film travel. The light from the mirror ll then passes through a lens I! and impinges upon a slit mask II as light images I! and I, the mask l3 having a slit l4 therein. The light passing through the slit is then projected by objective lenses I'I upon a film ll, the film being advanced. in any suitable manner well known in the art. This system is representative of the basic method of variable area push-pull recording.

Slit mask I3 is shown in greater detail in Fig. 2 in which the light images I! and I. are shown positioned thereon at times of no signal. Certain dimensions have been placed on the images although these dimensions are actually the dimensions oi. the apertures 8 and 9 of mask 1 and may not necessarily be those of the images, thediscrepancy depending upon the lens III and ii. The width of the widest portions of the apertures is 224 mils and that of the narrow tail portions, two mils. To obtain 100% modulation, the images must be deflected mils, which is the distance from the widest point of the apertures to the center of the slit l4, both apertures being identical in dimensions. It is to be noted that each aperture or image might be considered as 05 comprising two triangles, the larger triangles having their apexes coincident with the center of the slit as shown by the extended dotted lines of one side of the triangles. These two images alone could be used for recording a pure class B sound record. The remaining portions of the images are also triangles having a base dimension of 2b and an altitude of a plus 2 mils, since the tailis of 2 mils width. These dimensions have been designated as variable, since they may be u of any value depending upon the result desired, as will be shown hereinafter.

The record produced by the apertures just described is shown in Fig. 3, wherein for low modulations or image deflections not greater than the dimension 1), shown at 20, each light image produces a complete and continuous trace upon the film, these traces being mutually 180 out of phase. This is a pure push-pull class A track. For larger amplitudes and greater deflections, however, a combination push-pull class A and class B sound record is produced as shown in the remaining portion of the sound track of Fig. 3, each trace now being discontinuous and the portions thereof lying in mutually exclusive areas transversely of the film or traces.

Although one particular configuration of apertures is shown in Figs. 1 and 2, the apertures shown in Figs. 5 to '7 inclusive, shown positioned on the slit mask for zero signal, are of this same combination type and will also produce a track in accordance with the invention if constructed in the manner now to be described.

In Fig. 2 the light images are shown positioned with respect to the slit for a nul position of the galvanometer, the slit passing a light beam of a definite width. In Fig. 8 there is shown how the width of the beam varies for diflerent deflections of the galvanometer from its nul position, both negative and positive. The solid line l5 pertains to image i5, and the solid line [6' pertains to image l6 so that the length of the ordinate oc is the width of the beam l5 at no signal, and the ordinate o-d is the width of beam l6 under the same conditions, the width of both beams being c-d. For beam l5, no light will pass through the slit when the beam is deflected beyond a point e, while point I of the abscissa represents the no-light point for beam l6 except for the light passing through the tails of the images, which remains constant for all deflections beyond points e and f. Thus, for deflections or modulations between points e and f, light from both beams passes to the film and a pure push-pull class A record results, as shown at 2|] in Fig. 3.

It is to be noted from the above that, between the limits of e and f, the variation in lengths of the beams passing to the film is directly proportional to image deflection. Beyond these limits, there is also a constant proportionality but in a diiferent ratio. Since each light beam becomes ineffective at respective points e and 1' from the modulation standpoint, the active beam must increase its effectiveness sufliciently to compensate for the loss of the inactive beam. Thus it has been found that-the width of each beam must increase twice as fast for deflections greater than oe or of. This is shown in Fig. 8 by the increase in slope of lines l5 and Hi from points g and h respectively, to the points of 100% modulation or full deflection of the galvanometer or light beams. These steeper portions of curves l5 and It must have the same slope, such that they form continuations of each other as shown by the interconnecting dotted line g7i. Therefore, no matter what the particular configuration of the mask, as long as this relationship is produced in the length of the beam or beams striking the film, there will be no loss of amplitude or distortion at the translation point between pure push-pull class A and combination push-pull class A and B.

Thus, the apertures of Figs. 5 to '7, inclusive, are constructed in accordance with this principle, and any one, therefore, will produce a track in accordance with the invention. It is also to be understood that other push-pull aperture conflgurations may be employed which will be included within the principles of the invention. By this construction. therefore, there is provided an all push-pull recording system and all push-pull sound track having for low amplitudes the advantages of no azimuth distortion, substantially no process distortion for all frequencies, no unbalance distortion, and for higher amplitudes, materially reduced distortion due to faulty azimuth setting, faulty processing and faulty adjustment of the reproducing system. The system and track are also substantially noiseless, thus eliminating noise reduction mechanisms.

Since the point of transition from pure pushpull class A to combination push-pull class A and B may be variable, depending upon the values of the dimensions a and b in Fig. 2, the following table sets forth the amplitude levels at which changeover occurs as well as the variations in ground noise reduction for diflerent values of a and b:

Db. End 0! on n n+2 b Him in G. N. R. class A mm 0 2 0 35 100% 00 0. 31 4 6 2 26 28 0. 94 6 8 3 23 25 l. 25 8 1O 4 21 22 l. 56 10 12 5 20 2l. 1. 87 l2 l4 6 l8. 2 -19 2. 20 l4 l6 7 17. 1 l8 2. 50 l6 l8 8 l6. 0 16. 8 2. 8O 18 20 9 l5. 0 15. 6 3. 18 20 22 l0 l4. 0 14. 8 3. 44 22 24 11 13. 5 14. 0 3. 75 24 26 12 12. 8 l3. 2 4. 07 26 28 13 12. 2 l2. 5 4. 38 28 30 14 ll. 5 12 4. 70

With the values of a and I) both zero, it will be observed that a pure class B record is obtainable with its practical disadvantages as pointed out above. However, from the above table suitable values of the dimension 1: and b may be chosen to provide any desired transition point. This table is based upon apertures of 55 mils for full beam deflection, it being understood that all the values of the table would vary for other dimensions of the apertures.

To further illustrate the variation in distortion between a pure class B and the present invention, reference is made to Figs. 9 and 10, Fig. 9 showing by the solid line the push-pull class AB type of recording, while the entire curved line, a portion of which is dotted, shows the distortion in the class B recording system. The ordinates show percentage distortion, and the abscissas show amplitude of modulation in dbs., the curves being based on calculated data using a square-topped wave for simplicity.

It will be seen from Fig. 9, illustrating distortion variations as a function of amplitude modulation for a fixed azimuth rotation of 3.5 minutes from normal, that for 100% modulation or zero db. a very small amount of distortion occurs for both class B and push-pull class AB, the amount of distortion increasing as the amplitude decreases to the point of transition, which has been arbitrarily chosen at 20 db., at which point the distortion drops to zero for combination push-pull class A and B, while it continues to rise for pure class B. i

In Fig. 10, variations in distortion as a func-- tion of amplitude modulations for a fixed unbal-- ance in the reproducing system .are shown. In

this graph the horizontal dotted line represents the class B type of recording, while the solid line is representative of the push-pull class A-B type. Interpreting these curves, it is seen that the amount of distortion due to unbalance remains constant for all amplitudes or levels in the class B type and that, for 100% modulation or zero db., class A-B is slightly less than class B. For lower amplitudes of class AB, however, the amount of distortion decreases, as shown by the curve, to the arbitrarily chosen point of transition at 20 db., where only push-pull class A is eifective and the distortion is zero.

It is recognized that the distortion of squaretopped waves produced by faulty aperture azimuth adjustments or unbalance in the reproducing system will result only in amplitude distortion and, therefore, the values of distortion shown in Figs. 9 and 10 do not apply to the practical cases of sine waves or the complex waves of speech and music. However, the curves do illustrate the general diiferencein the amounts of distortion produced between class B and pushpull class AB systems and sound tracks and are shown solely for that purpose. However, data obtained from distortion experiments using pure sine waves substantially corroborate these differences in the amounts of distortion shown in Figs. 9 and 10.

From these curves, shown in Figs. 9 and 10, it is seen that less distortion is produced in the combination push-pull class AB system than in pure class B, while at the same time eliminating the use of noise reduction shutters necessary for the usual class A system. -With this type of invention, therefore, an entirely practical system of push-pull recording and reproduction is obtainable not subject to the critical adjustments required for the class B systems, while eliminating the use of noise reduction equipment now required for class A.

Now referring to Fig. 4, showing a mask suitable for combination standard class A and class B recording, two similar but unequal apertures 22 and 23 are illustrated as superimposed upon a'mask 24 having a slit 25 therein. Assuming the apertures are light images impinging on the mask, the images are shown for zero or normal position at times of no signal. This type of meal: will produce a standard class A record for signal amplitudes which do not produce deflections of the images greater than the distance i inasmuch as only image 22 will then pass light to the film, and the usual single-track record will be produced. For greater deflections thani, aperture 23 also functions, with the result that a combination standard class Aclass B record is produced, such a record being reproducible on a push-pull reproducing system since a photocell output is dependent solely upon change in light quanta within the operative limits of its characteristic.

Combination recording systems of the standard class Aclass B type have substantially all the advantages set forth above in connection with the all push-pull combination system. There are certain conditions, however. which must be fulfilled in the construction of the apertures of the standard class A combination. Since. only one aperture is functioning for pure class A during the recording of low amplitudes, the rate of change in light quanta at the point of changeover to the combination must remain the same as for the class A. Thus, referring to Fig. 4, the angle is the equal to angle 0, or, in broader terms, the increase in the length of one beam impressed on the film must be directly proportional to the decrease in length of the other beam. This is the case for the all push-pull combination also except that both beams are functionlnl during pure class A recording in the all push-pull v system.

Although the above invention has been disclosed applied to a variable area system, it is to be understood that for variable density systems, the fundamental principle is also applicable. That is, instead of the length or lengths of the impressed beam or beams varying as above disclosed, in the variable density system the densities should vary in this manner.

What I claim as my invention is:

1. A sound recording system comprising means for obtaining a plurality of light beams, and means for varying the light values of said beams in accordance with a signal, said light beams varying in value as class A for amplitude of said signal within a certain amplitude range of said signal, and varying in value as combination pushpull class A and B- for another amplitude range of said signal, the variations in said light beam corresponding to the amplitudes of said signal at all amplitudes thereof.

2. A sound recording system comprising a plurality of light beams, means for vibrating said beams in accordance with a signal to be recorded, and a film upon which said light beams are impressed, said light beams having a size and shape to produce a combination class A and push-pull class B sound record on said film having variations proportional to all amplitudes of said signal.

3. In combination, a source of light, means for producing a plurality of light beams from said source, a light-defining slit, means intermediate said beam producing means and said slit for vibrating said beams transversely of said slit in accordance with a signal, and a film upon which the emergent light beams from said slit are projected, said light beams having a size and shape such that for amplitudes of vibration up to a predetermined point said emergent beams produce a class A track upon said film, and for amplitudes of vibration above said predetermined point said emergent light beams produce a combination class Aclass B track upon said film, the variations in said light beams corresponding in a linear manner to all the amplitudes of said signal.

4. A recording system comprising a light source, means for defining a pair of light beams, a slit, and means for projecting said light beams in phase opposition on said slit, said light beams each having a size and shape such that vibrations thereof transversely of said slit in accordance with a signal increases and decreases the light of each light beam passing longitudinally through said slit, at a certain rate up to a predetermined amplitude of vibration, and at a different rate for amplitudes above said predetermined amplitude of vibration, said vibrations increasing and decreasing the light of both light beams in a linear manner for all amplitudes of said signal.

5. In a variable area sound recording system, means for producing a plurality of light beams, means for projecting said beams upon a longitudinal slit, a film having a motion transversely of said slit, and means for vibrating said light beams in the direction of movement of said film, said light beams having a shape and so positioned with respect to said slit that parallel beams are projected simultaneously on said film for vibrations of said beams up to a predetermined amplitude of vibration, and separate beams are projected at mutually exclusive intervals on said film for amplitudes above said predetermined amplitude point of vibration, the amount of said beams impinging upon said film being proportional at all times to a signal being recorded.

6. A light-defining mask having a pair of substantially triangular-shaped apertures therein horizontally disposed with respect to one another and oppositely opposed vertically, the width of said apertures in a vertical direction increasing in a predetermined proportion over a certain length and varying in a different proportion over the remainder of their length.

'7. A light-defining mask in accordance with claim 6 in which the increase in width of the remaining length of said apertures is twice as great as the increase in width over the other portion of said slit.

8. A film sound track having two substantially parallel continuous traces extending longitudinally of the film for signal amplitudes below a certain predetermined amplitude level and having two substantially parallel discontinuous traces for amplitudes above said predetermined level, the portions of said discontinuous traces lying in mutually exclusive areas transversely of said traces, and the modulations in said traces corresponding in a linear manner to all amplitudes of said signal.

9. A push-pull film sound track having traces, each trace containing continuous modulations for amplitudes below a predetermined level, and each trace containing discontinuous modulations for amplitudes above said level, all of said modulations being proportional to the amplitudes of a recorded signal.

10. A sound track having parallel traces for all amplitudes of signal, the modulations being continuous in at least one of said traces for amplitudes below a predetermined point, and discontinuous in both of said traces for amplitudes above said point, all of said modulations corresponding to the respective amplitudes of the recorded signal.

11. In a sound recording system, the method of recording a sound track which comprises recording amplitudes below a certain level as class A, and amplitudes above a certain level as combination class A and class B, all ofsaid amplitudes corresponding to the respective amplitudes of a recorded signal.

12. In a sound recording system the method of producing a combination class A and class B sound record comprising recording signals below a certain predetermined level as pure class A signals, and all amplitudes of said signals above said certain level as combination class A and class B, all recorded amplitudes corresponding in a linear manner to all amplitudes of said signals.

13. In a push-pull variable area sound recording system the method of recording a combination class A and class B sound track comprising simultaneously varying the lengths of horizontally spaced light beams such that the decrease in length of one beam is proportional to the increase in length of the other oil said beams for signals up to a predetermined amplitude, and for signals above said predetermined amplitude, the variations in the length of each 01 said beams is at twice the rate as the variations in amplitudes below said predetermined amplitude.

14. A push-pull variable area sound system in accordance with claim 13 in which the variations in the length of said light beams for signal having amplitude above said predetermined level occurs at mutually exclusive time intervals.

15. In a sound system an aperture mask having a plurality of apertures therein, said apertures being oppositely disposed vertically and spaced'horizontally from one another, a source of light for projection through said apertures to form light beams, and a slit mask upon which said light is projected, one of said light beams having its tip normally spaced a distance from the slit in said mask substantially equal to the distance from the tip of said other light beam to the point where said slit intercepts said other light beam.

16. A combination class A and class B sound track having a single continuous trace for signal amplitudes within a predetermined range and a plurality of discontinuous modulated traces for signal amplitudes in another range, all of said amplitudes being proportional to corresponding amplitudes of the signal represented thereby.

1'7. A sound track having a plurality of traces, at least one of which contains the complete variations of a recorded signal for amplitudes below a predetermined amplitude value, said trace containing only a portion of the variations of said recorded signal for amplitudes above said predetermined amplitude value, the remaining portion of said variations of said recorded signal during the amplitude range above said predetermined amplitude value being contained in said other trace or traces.

C. HAWLEY CARTWRIGHT. 

